With an increase in integration of IC circuits, heat value is increased in electronic parts such as hybrid packages, multimodules, and sealed type integrated circuits with plastics and metals. Since the increase of temperature due to the increased heat value may cause malfunction of the electronic parts, a countermeasure for preventing the malfunction by attaching a heat-radiating member such as a heat sink to the electronic part has been taken.
As a method for providing electronic parts with heat-radiating members, a method which comprises using an adhesive prepared by adding aluminum powder etc. to a composition comprising a polymerizable acrylate monomer and a free radical initiator is proposed in U.S. Pat. No. 4,722,960. In this method, after the adhesive is applied onto one or both of the electronic part and the heat-radiating member, it is necessary to effect a curing treatment using a primer or blocking oxygen. Such an adhesion treatment needs a long time and much labor, and the materials to be adhered have to be fixed temporally until the completion of curing, thereby resulting in poor efficiency in producing the electronic devices.
In contrast thereto, it is proposed to use so-called heat-conductive and pressure-sensitive adhesive having both heat-conductivity and pressure-sensitivity. By using these heat-conductive and pressure-sensitive adhesives, electronic parts can be easily provided with heat-radiating members while needing neither a long time nor much labor. Since heat-radiating plates and semiconductor packages are made of metals or metal oxides in a highly polar surface state, it is necessary in this method to employ pressure-sensitive adhesives being highly adhesive thereto and showing excellent heat conductivity. However, few of the existing heat-conductive and pressure-sensitive adhesives can satisfy these requirements.
In acrylic pressure-sensitive adhesives, for example, it is expected that the adhesive strength to heat-radiating plates or semiconductor packages can be improved by using highly polar monomers (acrylic acid, etc.) in the step of synthesizing adhesive polymers. However, most of highly polar monomers have glass transition points, as a homopolymer, at ordinary temperature (20.degree. C.) or higher. When such a monomer is used in a large amount, therefore, the glass transition point of the resultant polymer is elevated and thus the modulus of elasticity thereof is elevated at ordinary temperature at which adhesives are used in general. This increase in the modulus of elasticity prevents the adhesive from acquiring a contact area with a heat-radiating plate or a semiconductor package and thus adhesion should be performed under elevated pressure or at a high temperature. However, semiconductor packages in recent years, which have become highly integrated and delicate, frequently suffer from pin breakage or destruction of the packages per se during adhesion under high pressure. When adhesion is performed under low pressure so as to avoid these phenomena, any sufficient contact area can not be obtained and consequently the heat-conductivity and the adhesion are lowered, thus inducing malfunction or fall-off.
To overcome these problems, attempts are made to reduce the amount of highly polar monomers (acrylic acid, etc.) in the synthesis of adhesive polymers to thereby secure a sufficiently large contact area. In this case, however, the resultant adhesive shows a lowered adhesive strength to a heat-radiating plate or a semiconductor package and, as a result, suffers from the problems of fall-off, etc. too. In heat-conductive and pressure-sensitive adhesives of this type, it is a practice to add heat-conductive fillers to pressure-sensitive adhesives to thereby achieve high heat-conductivity. When these heat-conductive fillers are added in a small amount, the desired high heat-conductivity can be scarcely established. When these heat-conductive fillers are added in a large amount so as to improve the heat-conductivity, on the other hand, the adhesive strength to a heat-radiating plate, etc. is lowered and, as a result, there arise problems of fall-off, etc.
As described above, it is advantageous to use heat-conductive and pressure-sensitive adhesives since electronic parts can be easily provided with heat-radiating members thereby without needing a long time or much labor. However, there have been known few heat-conductive and pressure-sensitive adhesives so far which are highly adhesive to highly polar heat-radiating plates or semiconductor packages and have high heat-conductivity. Accordingly, it cannot be always concluded that malfunctions of electronic parts caused by temperature increase can be effectively prevented by providing heat-radiating members.
Under these circumstances, the present invention aims at providing heat-conductive and pressure-sensitive adhesives being excellent in adhesion and heat-conductivity and adhesive sheets thereof, more particularly, heat-conductive and pressure-sensitive adhesives by which heat-radiating members can be adhered and fixed to electronic parts while achieving a high heat-conductivity, even though adhesion is performed under such low pressure as to cause no breakage of the electronic parts, and thus the heat evolved in the electronic parts can be efficiently transferred to the heat-radiating members to thereby prevent malfunctions of the electronic parts caused by temperature increase without fail, and adhesive sheets with the use of these heat-conductive and pressure-sensitive adhesives.